1. Field of the Invention
The present invention relates to a computer monitoring device for monitoring whether a normal operation is being carried out by a computer applied to control upward and downward movements of a window in a power window system for a vehicle, and detection of nipping of foreign matter by the window, or the like.
2. Description of the Related Art
Conventionally, in a system using a microcomputer, a power window system for a vehicle or the like, the microcomputer is operated by a stabilized power supply, which is controlled to have a constant voltage on the basis of a voltage energized from a battery (about 5 V). At this point, in order to prevent the microcomputer from malfunctioning (for example, an operation that causes a power window to operate contrary to the purpose of a switching operation due to a latch-up or the like of an active signal, or the like), when the microcomputer is in an abnormal state, an operational state of the microcomputer is monitored by a microcomputer monitoring circuit (i.e., a so-called watchdog timer circuit).
Specifically, when the microcomputer is operating normally, it outputs a pulse signal to the microcomputer monitoring circuit in a predetermined cycle. While this pulse signal is being detected by the microcomputer monitoring circuit, the microcomputer is thereby judged to be operating normally. When the microcomputer monitoring circuit cannot detect the pulse signal for a predetermined period of time or more, the microcomputer monitoring circuit assesses that the microcomputer is in an abnormal state. Then, the microcomputer monitoring circuit outputs a signal for prohibiting signal processing circuit from communicating with the microcomputer to the signal processing circuit, which outputs a signal for operating a power window, so as to prevent the power window from malfunctioning. Then, the microcomputer monitoring circuit outputs a reset signal to the microcomputer so as to restart the microcomputer.
In this way, it is ensured that the microcomputer operates safely when the microcomputer is in an abnormal state. However, if the power source supplied to the microcomputer monitoring circuit is also employed for the power source supplied to the microcomputer, when the power supply voltage supplied to the microcomputer decreases, the power supply voltage supplied to the microcomputer monitoring circuit also decreases. As a result, there arises a problem in that it is no longer possible to ensure safe operation for preventing the power window from malfunctioning or the like when the microcomputer is in an abnormal state.